1. Field of the Invention
The present invention relates to a method and apparatus for controlling an air fuel ratio by using a gas concentration sensor in a fuel supply system for an internal combustion engine, particularly to a method and apparatus for measuring a gas concentration of a combustible gas such as a gasoline vapor gas purged from a canister and mixed with an intake air that is supplied to an intake manifold of an internal combustion engine cylinder by opening a throttle valve, the sensor being placed between the engine cylinder and an inlet for the vapor gas that is purged from the canister and introduced downstream of the throttle valve.
2. Related Art
Conventionally, a fuel supply system for supplying fuel from a fuel tank to the internal combustion engine includes a first supply system which functions in a manner that a fuel liquid such as gasoline is pumped from the fuel tank through a fuel pipe to a fuel injector fixed to an internal combustion engine cylinder by means of a fuel pump.
The fuel supply system further includes a second supply system which functions in a manner that a fuel vapor generated in the fuel tank is temporarily adsorbed or stored by a canister and then the fuel vapor is purged by the canister and sent as a purge gas to the intake manifold so as to be mixed with an intake air entering from the throttle valve.
In other words, in addition to the fuel liquid injected from the injector to the engine cylinder, the fuel vapor vaporized from the fuel tank is purged and mixed with the intake air to form a mixture gas that injects the fuel liquid to be atomized in the engine cylinder. Such fuel vapor vaporized in the fuel tank may be hereinafter referred to as xe2x80x98purge gasxe2x80x99.
3. Problems to be Solved by the Invention
As a result of supply to the engine of the purge gas in addition to the injected fuel, an air-fuel ratio maintained or determined by a throttle valve, which ratio is controlled based on an injection amount of the fuel is affected or rather deviated from a target value such as a theoretical one, lowering a purification capability of a three-way catalytic exhaust gas purification system for reducing harmful gasses such as CO, HC and NOx.
Further, when the purge gas is used as a main portion of fuel for combustion at the time of starting up the engine and/or when a three-way catalytic converter of the exhaust gas purification system is in an inactive state due to a cold weather, a misfire or incomplete combustion occurs, unless the purge gas concentration is measured with high accuracy and the supply amount of the purge gas is controlled optimally.
Various sensors using surface-characteristics such as an ultrasonic sensor and an oxide semiconductor sensor having electrodes on its surface have been proposed for measuring concentration of such a purge gas contained in the mixture gas before the mixture gas atomizes the liquid fuel by the fuel injector. However, a satisfactory sensor for accurate measurement of the purge gas amount or concentration have not been developed yet, because in addition to variation in output of such a sensor over time, output errors due to adhesion of foreign substances to a surface of the ultrasonic sensor and/or due to moisture and miscellaneous gases contained in the air taken in have always hindered accurate measurement of the purge gas concentration.
Therefore, it has been difficult that a precise or accurate measurement of the purge gas concentration of the gas mixture made of the purge gas and the intake air flowing in the intake manifold or an intake pipe before the liquid fuel is atomized with the gas mixture by fuel injector or before being injected into the engine cylinder.
The present invention has been accomplished in order to solve the above-described problems.
An object of the present invention is to provide a method and apparatus using a gas concentration sensor for measuring concentration a fuel vapor (or rather purge gas) mixed in an intake air, by which method and apparatus an accurate measurement of concentration of such a fuel-vaporized purge gas entering an intake manifold of an internal combustion engine is attained.
Another object of the present invention is to provide a fuel supply system using a gas concentration sensor for accurately controlling an air fuel ratio for an internal combustion engine, the sensor being disposed in an intake manifold (or an air-intake pipe) which is located between a throttle valve and a fuel injector of the internal combustion engine.
A feature of a method and apparatus using a gas concentration sensor for accurately measuring the vapor gas according to the invention is that before the fuel-vaporized gas purged from the canister enters into the intake manifold whereat the sensor detects the gas concentration of the purged gas, the sensor is adjusted so as to read a zero point (or zero output level) of the sensor output. In other words, when the purge gas stops its flow from the canister, namely when the intake pipe or manifold evacuates the purge gas so that an ambient air enters the intake pipe or manifold, the sensor output is offset reading a zero output of the sensor for measuring the concentration of the specific fuel vaporized gas purged.
Specifically, the present invention provides a method and apparatus using a gas concentration sensor for measuring concentration of a purge gas (i.e. vapor gas vaporized from a liquid fuel) purged and contained in a mixture gas comprised of the purge gas and an ambient air before the mixture gas atomizes the liquid fuel by a fuel injector: wherein the gas concentration sensor is disposed in an intake manifold of an internal combustion engine or a pipe connected to the intake manifold so as to measure concentration of the purge gas; and wherein the concentration of the purge gas is determined by comparing two output values of the gas concentration sensor, the one output value being measured before the purge gas is supplied into the intake manifold or pipe while the other output value being measured after the purge gas is supplied into the intake manifold or pipe. In one aspect of the invention, a degree of degradation or rather deterioration of the gas concentration sensor is pre-checked when the sensor outputs the concentration of the purge gas for computing the concentration. In other words, a zero-point correction of the gas concentration sensor is renewed every time when the purge gas is evacuated from the intake manifold wherein the sensor is placed.
In the present invention, during that the specific purge gas is not supplied from the canister and therefore the concentration of the purge gas to be mixed with the intake air that enters into the fuel injector for atomization of the liquid fuel is supposed to be zero, the output of the gas concentration sensor is preset or offset to be a zero level output(V0) corresponding to a relative gas concentration of 0%.
In actual use, the gas concentration sensor is affected not only by its variation over time but also miscellaneous gases and/or foreign substances including moisture of the air taken in or evaporating from the liquid fuel so that the sensor output often deviates from the preset value (V0) even when the purge gas concentration is 0%. When the sensor output shows a deviation from V0 that corresponds to the specific purge gas concentration of 0%, a certain shift amount (xcex94V) namely a subtraction V0 from the sensor output V1 (V1xe2x88x92V0=xcex94V) represents either a true value of concentration of the purge gas in the mixture gas or a degraded or deteriorated performance of the sensor due to such variation or influence.
Therefore, the method and apparatus using a gas concentration sensor for measuring the purge gas according to the invention is characterized in that zero-point correction of the sensor output is performed before the sensor output is subjected to computing the purge gas concentration, regardless that the gas concentration sensor has undergone deterioration or deterioration or the like, so as to minimize an offset correction amount.
For example, the sensor output V1 seemingly caused by variation over time or other causes is always subtracted by the shift amount xcex94V which is renewed or optimized by the latest offset zero point value V0. (The shift amount xcex94V is the latest sensor output V1 subtracted by the V0 measured latest.) Thus, the zero point level V0 of the concentration of the gas sensor to be used for reference is corrected with the newest or latest measured data of V1 that is measured when the purge gas is evacuated from the intake manifold, according to the invention.
In an aspect of the invention, xcex94V is taken into consideration in order to eliminate influences of variation over time and the like. V0 (or V1) is an offset output value of the gas concentration sensor when the specific purge gas is absent and the zero-point correction for the sensor is performed on the basis of such V0 and V1.
An alternative way for the zero-point correction may be done by using a mean value that is computed with the at least two offset values V0s (or V1s). An advantage of this alternative way is that when the gas concentration sensor outputs an abruptly varied signal of V0 at one time, an air fuel ratio controller will only use an average value of plural V0 values so as to avoid any risk of such abrupt zero-point reference change.
When the concentration of the specific purge gas is introduced for measurement, the gas concentration is determined by an output value V2(out putted from the sensor) subtracted by the offset value V0 (zero point value) that is corrected by the latest measured offset value (V1).
A second alternative way may be that only when the previously determined offset value (V0) of the gas concentration sensor corresponding to the gas concentration of zero is lower than a certain offset output value (V1)(i.e. when V1 greater than V0) and when the sensor output (V2) corresponding to the specific purge gas concentration becomes higher by xcex94V (=V1xe2x88x92V0), the difference (V2xe2x88x92xcex94V) is used as a value corresponding to the actual purge gas concentration. The thus determined gas concentration by referring to the output value (V2xe2x88x92xcex94V) of the sensor may be advantageously assisted by use of a memory media recording a map wherein the sensor output is illustrated as a function of the actual purge gas concentration of the mixture gas.
The output of the deteriorated gas concentration sensor may be corrected by a plurality of the maps or selectively corrected in accordance with the correction amount.
Once the concentration of the specific purge gas is stably obtained through the zero-point correction, supply of the specific purge gas can be measured precisely for control of the air fuel ratio and/or post-combustion air-fuel ratio control.
Further, a control of the supply of the specific purge gas can be much easily done, according to a method and apparatus of the present invention, if an additional step in which the opening of the purge valve is controlled on the basis of the sensor output so as to regulate the supply (or flow rate) of the purge gas.
Types of the specific purge gases may include a combustible gas such as fuel vapor supplied from a canister into the intake air and unburned gas from an EGR system.
In an aspect of the invention, the method and apparatus using a gas concentration sensor for accurately measuring the purge gas concentration may be characterized in that the offset output of gas concentration sensor output is detected and adjusted during cranking immediately after an ignition key is turned and before the liquid fuel is atomized and injected into the engine cylinder.
An important feature is a timing at which the output of the gas concentration sensor is detected for zero point offset of the sensor. The best timing is during when the intake air serving as a base gas for the air fuel ratio is mixed with no specific purge gas. Such timing is during a cranking period for an engine or during the start-up period in which the activity of the catalytic converter is low. The zero-point correction in which the shift amount of the sensor output is taken into consideration is preferably performed immediately after detection of any degradation of the sensor. It is preferable that the zero-point correction is performed at any time upon measurement of actual gas concentration before the vapor gas is purged from a canister. It may be best to adjust the offset zero point level during a purging operation just after the ignition key is turned.
When the purging operation is first performed after the ignition key is turned, influence of fuel adhering to a purge gas supply passage on the downstream side with respect to the purge valve can be eliminated so as to determine the degree of the deterioration of the gas concentration sensor more accurately, and to perform the zero-point correction more accurately.
The invention also teaches a timing at which the deterioration or degradation of the gas concentration sensor is detected. Such a timing is during a fuel-cut operation in which purging is not performed and only the base gas of the air is taken in.
The detection of the degree of the deterioration of the gas concentration sensor may be performed while a signal from an ECU or one of signals from various sensors mounted on the vehicle is used as a trigger signal.
An ultrasonic sensor using ultrasonic waves such that their speed changes in accordance with the concentration of the specific purge gas in the intake air may be preferably utilized as the gas concentration sensor used in the method and apparatus of the invention. In such a ultrasonic sensor, the ultrasonic waves are transmitted against the intake air and a propagation speed (i.e. propagation time) of the ultrasonic waves is detected in order to determine the concentration of the specific purge gas.
Another type of the gas concentration sensor usable in the method and apparatus of the invention comprises an element formed of oxide semiconductor such as tin oxide and utilizes a phenomenon that the amount of absorbed oxygen on the element surface changes due to oxidation of the specific purge gas on the element surface. A surface electrical resistance of the element changes due to variation in the number of free electrons. The concentration of the specific purge gas is determined through detection of variation in the electrical resistance by electrodes formed on the element surface.
Another sensor for this use may include an element formed of solid electrolyte, in which the concentration of the specific purge gas is determined through detection of a mixed electrode potential relative to a reference electrode, through reaction between the specific purge gas and catalyst coated on the element surface, or through a potential difference which varies according to the Nernst equation.
Other gas concentration sensor possibly utilized for the present invention may be a heater type using e.g. a platinum wire, wherein the amount of heat generated due to oxidation of the specific purge gas on the surface of the platinum wire is detected on the basis of variation in the electrical resistance of the platinum wire so that the concentration of the specific purge gas is determined. Other type of the sensor suitable for the present invention may utilize an electrostatic capacity measured between paired electrodes disposed in a measurement chamber into which the specific purge gas is introduced. The concentration of the specific purge gas is determined through detection of the electrostatic capacity between the paired electrodes at the time when the specific purge gas is introduced.
The present invention provides a controller for an air fuel ratio of an internal combustion engine using the gas concentration sensor, comprising: memory means for storing, as a reference output, an output of the gas concentration sensor when the concentration of the specific purge gas is 0%; actual output detection means for detecting an actual output of the gas concentration sensor on the basis of an output of the gas concentration sensor before the specific purge gas is supplied; and zero-point correction means for comparing the reference output and the actual output in order to correct the zero point of the gas concentration sensor. In this controller according to the invention, the memory means stores therein a reference output-which is an output of the gas concentration sensor when the concentration of the specific purge gas is 0%. The actual output detection means detects an actual output of the gas concentration sensor corresponding to the 0% concentration, on the basis of an output of the gas concentration sensor before the specific purge gas is supplied. Subsequently, the zero-point correction means compares the reference output and the actual output in order to determine the difference therebetween, on the basis of which the zero point of the gas concentration sensor is corrected.
The controller may further comprise a measurement means for determining the concentration of the specific purge gas, on the basis of the output of the gas concentration sensor subjected to the zero-point correction. The concentration of the specific purge gas is determined on the basis of the output of the gas concentration sensor after being subjected to the zero-point correction. Therefore, the concentration of the specific purge gas can be determined accurately, while influences of variation over time and the like are eliminated.
The controller may further comprise an adjustment means (e.g. a purge valve) for adjusting supply of the specific purge gas on the basis of the output of the gas concentration sensor subjected to the zero-point correction. The gas concentration can be measured precisely on the basis of the output of the gas concentration sensor after being subjected to the zero-point correction. Therefore, a required amount of the specific purge gas can be supplied accurately through control of adjustment means performed on the based of the precise gas concentration.